The present invention relates to a process for preparing alkylpyrylium salts, containing at least three alkyl groups in the 2-, 4- and 6-position, by reacting a branched alkane with an acylating agent in the presence of a Lewis acid.
The pyrylium salts represented by the general formula: ##STR1## wherein R' and R" each represent alkyl, each R"' represents hydrogen or alkyl, and Y.sup.- represents an acid anion, are compounds which are known in the art and are of very great industrial value in various respects. Thus, they are used as adjuvants in photosensitive compositions employed in photography (see, e.g., French Pat. No. 1,387,433 and U.S. Pat. No. 3,250,615). Furthermore, pyrylium salts constitute intermediates which are in great demand in organic synthesis. Thus, reaction of pyrylium salts with ammonia or amines, leads to the corresponding pyridine bases. By the action of an alkali metal base such as sodium hydroxide, trisubstituted pyrylium salts containing a methyl group in the 2-position are converted into disubstituted phenols. Apart from these reactions which are of particular importance from the industrial point of view, pyrylium salts are applied in numerous other ways in organic synthesis (see, e.g., S. V. KRIVUN et al., Russ. Chem. Rev., 43, 835-850 (1974)).
The synthesis of pyrylium salts has been the subject of numerous works (compare A. T. BALABAN, Advances in Heterocyclic Chemistry volume 10, pages 214-326 (1969)). Among the various processes for preparing the above alkylpyrylium salts, the most valuable industrially are those which employ diacylation of olefins or their precursors such as tertiary alkyl halides or tertiary or secondary alcohols or their esters, in the presence of Lewis acids such as the halides of various metals (for example AlCl.sub.3, FeCl.sub.3, SbCl.sub.5, SnCl.sub.4 or ZnCl.sub.2) or boron trifluoride or its complexes with alkyl ethers (ethyl or isopropyl ether) or also in the presence of a strong Bronsted acid such as perchloric acid or trifluoromethanesulphonic acid. Acylating agents used are acid anhydrides and more particularly the halides of aliphatic or aromatic acids. The diacylation of olefins can be presented schematically as follows: ##STR2## The yields of pyrylium salts vary according to the olefin (or its precursor), the acylating agent and the catalyst used (compare A. T. BALABAN et al., Ann., 625, pages 74 to 88 (1959), J. Chem. Soc., 1961, pages 3,553-3,561 and French Pat. Nos. 1,340,970 and 1,340,971).
Despite the value of the diacylation of olefins it appears to be desirable from the industrial point of view to have available a process for the preparation of pyrylium salts from branched alkanes or their mixtures such as the fractions of saturated aliphatic hydrocarbons which are available on an industrial scale. It is surprising to find that, although the diacylation of olefins or their precursors has formed the subject of numerous works since 1959, there are no publications on the preparation of pyrylium salts from branched alkanes. Indeed, H. HOPFF et al, (Ber., 69, pages 2,244 to 2,251 (1936)) have studied the reaction of acetyl chloride with saturated hydrocarbons, and in particular with isobutane, but they do not observe the formation of pyrylium salts.